Apparatus and method for allocating walsh codes to access terminals in an adaptive antenna array CDMA wireless network

ABSTRACT

An apparatus for allocating orthogonal codes used for downlink transmissions to wireless access terminals for use in a base station of a code division multiple access (CDMA) wireless network. The base station communicates with the wireless access terminals using transmit beams formed by an adaptive antenna array. The apparatus comprises: 1) a database for storing R active wireless terminal records, each of the R active wireless terminal records containing: a) an active orthogonal code and b) corresponding downlink beamforming coefficients used to communicate with one of the wireless access terminals; and 2) a controller associated with the database that receives a notification that a new wireless access terminal is accessing the base station and, in response to the notification, compares each of the R active wireless terminal records to new downlink beamforming coefficients associated with the new wireless access terminal. The controller determines at least one active wireless terminal record containing corresponding downlink beamforming coefficients that have the least correlation with the new downlink beamforming coefficients.

[0001] The present invention claims priority to U.S. ProvisionalApplication Serial No. 60/282,059 filed Apr. 6, 2001.

TECHNICAL FIELD OF THE INVENTION

[0002] The present invention is directed, in general, to digitalcommunication systems and, more specifically, to an apparatus and methodfor dynamic allocation of Walsh codes in an adaptive antenna array (AAA)CDMA base transceiver station (BTS) utilizing spatial diversity forcommunications links with mobile users to support a traffic channelcount greater than the Walsh code limit such as that found 2G (IS-95)systems, with a limit of 64, or in 3G (IS2000) systems, with limits ofeither 64 or 128.

BACKGROUND OF THE INVENTION

[0003] The radio frequency (RF) spectrum is a limited commodity. Only asmall portion of the spectrum can be assigned to each communicationsindustry. The assigned spectrum, therefore, must be used efficiently inorder to allow as many frequency users as possible to have access to thespectrum. Multiple access modulation techniques are some of the mostefficient techniques for utilizing the RF spectrum. Examples of suchmodulation techniques include time division multiple access (TDMA),frequency division multiple access (FDMA), and code division multipleaccess (CDMA).

[0004] CDMA modulation employs a spread spectrum technique for thetransmission of information. The CDMA wireless communications systemspreads the transmitted signal over a wide frequency band. Thisfrequency band is typically substantially wider than the minimumbandwidth required to transmit the signal. A signal having a bandwidthof only a few kilohertz can be spread over a bandwidth of more than amegahertz.

[0005] All of the wireless access terminals, including both mobilestations (e.g., cell phone) and fixed terminals, that communicate in aCDMA system transmit on the same frequency. In order for the basestation to identify the wireless access terminals, each wireless accessterminal is assigned a unique pseudo-random (PN) long spreading codethat identifies that particular wireless access terminal to the wirelessnetwork. Typically, each long code is generated using the electronicserial number (ESN) of each mobile station or fixed terminal. The ESNfor each wireless access terminal is unique to that wireless accessterminal.

[0006] Similarly, each sector of a base station uses a unique short code(containing 2¹⁵ bits) to identify itself to access terminals. Thosefamiliar with the art will recognize that a sector is defined by thecoverage provided by the pilot, paging and synch overhead channelstransmitted by the BTS for both non-adaptive and adaptive antennasystems.

[0007] In a preferred implementation, the user data to be transmitted toa wireless access terminal is first framed, convolutionally encoded,repeated, interleaved, and encoded with the long code to form a basebandsignal. The baseband signal is then separated into an in-phase (I)component and a quadrature (Q) component prior to quadrature modulationof an RF carrier and transmission. The I-component and Q-component arespread with a unique Walsh code of length M=2^(N) uniquely assigned toeach access terminal assigned to a traffic channel in the sector. TheI-component is modulated by a time-offset short pseudo-random noise(I-PN) binary code sequence derived from the short code of length 2¹⁵bits. The Q-component is modulated by a time-offset short pseudo-randomnoise (Q-PN) binary code sequence derived from the short code of length2¹⁵ bits. In an alternate embodiment, the quadrature binary sequence maybe offset by one-half of a binary chip time. Those skilled in the artwill recognize that the in-phase component and the quadrature componentare used for quadrature phase shift keying (QPSK) modulation of an RFcarrier prior to transmission.

[0008] The maximum capacity of a base transceiver station in a CDMAwireless network is limited by the number of unique orthogonal codes(Walsh codes) that are available for assignment to traffic channels ineach sector. The number of orthogonal codes available for trafficchannel assignment is limited to 56-61 for IS-95; to 56-61 for RadioConfiguration 1, 2 or 3 of IS-2000; and 119-125 for Radio Configuration4 or higher in IS-2000, depending on the number of paging channelsassigned. The codes allocated to traffic channels may support eithervoice or packet data services.

[0009] Those acquainted with the art will recognize that the number ofsimultaneous traffic channels supported over the RF links to wirelessaccess terminals depends on the propagation environment experienced bythe access terminals. For a typical, good propagation mobile environment(defined in the art as Vehicular B model), the EVRC capacity supportedon the forward and reverse RF links is approximately 24 Erlangs per CDMAcarrier per sector in a three-sector antenna configuration. A trafficload of 24 Erlangs corresponds to 34 EVRC traffic channels with a 1%blocking probability. With an average soft handoff capacity gain of 40%,this requires 48 Walsh codes per sector on the forward link. A handoffgain of 60%, which may occur in some dense urban or highly congestedareas, would require up to 54 Walsh codes.

[0010] For a wireless mobile application, the voice traffic capacity forEVRC vocoding may be as high is 65 Erlangs, or 80 traffic channels witha 1% blocking probability. For an adaptive antenna array basetransceiver subsystem, a capacity increase of two to four times (i.e.,2×to 4×) translates into a requirement for up to 192 Walsh codes for 40%soft handoff gain and up to 216 Walsh codes for 60% soft handoff gain.In a non-mobile, wireless application, up to 320 Walsh codes arerequired. Thus, there are numerous scenarios in which the number ofchannels supported over the air exceeds the limit of 64 available Walshcodes for Radio Configuration 3 or lower or 128 available Walsh codesfor Radio Configuration greater than 3.

[0011] Quasi-orthogonal codes have been used for increasing Walsh codeavailability. However, this technique results in degraded performanceand lower-than-expected RF capacity due to requirements for greaterEb/No at the receiver. Another prior art method includes a segmentationof the coverage area into six sectors in non-adaptive antenna systems,which allows greater Walsh code reuse. However, the result is greaterhandoff transitions and increased probability of dropped calls. Thosefamiliar with the art will recognize that doubling the number of sectorsdoes not allow a doubling of Walsh code reuse due to the number of codesrequired to support soft handoff and due to added overlap regions ofadjacent sector antenna patterns. However, this method is not applicablefor an adaptive antenna array base transceiver subsystem (BTS) in whichmultiple antennas and a baseband AAA processor module are employed persector.

[0012] Therefore, there is a need for improved CDMA wireless networks inwhich the number of users per sector is not limited by the number ofavailable Walsh codes. In particular, there is a need for a wirelessCDMA adaptive antenna array base station that can more efficiently usethe available Walsh codes by dynamically allocating Walsh codes in thebase station sectors so that a single Walsh code may be used tocommunicate simultaneously with two or more wireless access terminalswithin the same sector. More particularly, there is a need for a CDMAwireless base station that can dynamically allocate Walsh codes in beamsformed by adaptive antenna arrays of the base station so that a singleWalsh code may be used to communicate simultaneously with two or morewireless access terminals in the same sector.

SUMMARY OF THE INVENTION

[0013] The present invention provides an apparatus and method for usingthe spatial isolation provided by an adaptive antenna array to maximizethe re-use of Walsh codes in a base transceiver subsystem of a wirelessnetwork base station. This allows the BTS to support the full capacityof the air interface in adaptive antenna array operation so that thecapacity is not constrained by the 64 or 128 Walsh code limit.

[0014] To address the above-discussed deficiencies of the prior art, itis a primary object of the present invention to provide an apparatus forallocating orthogonal codes used for downlink transmissions to aplurality of wireless access terminals for use in a base station of acode division multiple access (CDMA) wireless network, wherein the basestation communicates with the plurality of wireless access terminalsusing transmit beams formed by an adaptive antenna array. According toan advantageous embodiment of the present invention, the apparatuscomprises: 1) a database capable of storing R active wireless terminalrecords, each of the R active wireless terminal records containing: a)an active orthogonal code and b) corresponding downlink beamformingcoefficients used to communicate with one of the wireless accessterminals; and 2) a controller associated with the database capable ofreceiving a notification that a new wireless access terminal isaccessing the base station and, in response to the notification,comparing the each of the R active wireless terminal records to newdownlink beamforming coefficients suitable for forming a downlinktransmit beam for transmitting to the new wireless access terminal and,in response to the comparison, determines at least one active wirelessterminal record containing corresponding downlink beamformingcoefficients that have the least correlation with the new downlinkbeamforming coefficients.

[0015] According to one embodiment of the present invention, thecontroller assigns an active orthogonal code in at least one activewireless terminal record to be used in downlink transmissions to the newwireless access terminal.

[0016] According to another embodiment of the present invention, thebase station uses up to K orthogonal codes for the downlinktransmissions and the controller compares each of the R active wirelessterminal records to the new downlink beamforming coefficients inresponse to a determination that all of the K orthogonal codes are inuse.

[0017] According to still another embodiment of the present invention,the controller determines a first plurality of active wireless terminalrecords containing corresponding downlink beamforming coefficients thathave the least correlation with the new downlink beamformingcoefficients and further determines from the first plurality of activewireless terminal records a first active wireless terminal recordcontaining an active orthogonal code used for downlink transmissions toa least number of the plurality of wireless access terminals.

[0018] According to yet another embodiment of the present invention, thecontroller assigns the active orthogonal code in the first activewireless terminal record to be used in downlink transmissions to the newwireless access terminal.

[0019] According to a further embodiment of the present invention, thebase station is operable to communicate in S sectors of a cell siteassociated with the base station and the base station uses up to Korthogonal codes in each of the S sectors for the downlink transmissionsand wherein the controller compares each of the R active wirelessterminal records to the new downlink beamforming coefficients inresponse to a determination that all of the K orthogonal codes are inuse in a first sector in which the new wireless access terminal isaccessing the base station.

[0020] According to a still further embodiment of the present invention,the controller determines a first plurality of active wireless terminalrecords containing corresponding downlink beamforming coefficients thathave the least correlation with the new downlink beamformingcoefficients and further determines from the first plurality of activewireless terminal records a first active wireless terminal recordcontaining an active orthogonal code used for downlink transmissions toa least number of the plurality of wireless access terminals.

[0021] According to a yet further embodiment of the present invention,the controller assigns the active orthogonal code in the first activewireless terminal record to be used in downlink transmissions to the newwireless access terminal.

[0022] In one embodiment of the present invention, the controllerreceives the new downlink beamforming coefficients from a beamformingcontroller that determines the new downlink beamforming coefficientsfrom an uplink signal transmitted by the new wireless is accessterminal.

[0023] In another embodiment of the present invention, the base stationis operable to communicate in S sectors of a cell site associated withthe base station and the new wireless access terminal is being handedoff from a first sector of the cell site to a second sector of the cellsite, wherein each of the R active wireless terminal records areassociated with the second sector and the controller receives the newdownlink beamforming coefficients from active wireless terminal recordsassociated with the first sector.

[0024] The foregoing has outlined rather broadly the features andtechnical advantages of the present invention so that those skilled inthe art may better understand the detailed description of the inventionthat follows. Additional features and advantages of the invention willbe described hereinafter that form the subject of the claims of theinvention. Those skilled in the art should appreciate that they mayreadily use the conception and the specific embodiment disclosed as abasis for modifying or designing other structures for carrying out thesame purposes of the present invention. Those skilled in the art shouldalso realize that such equivalent constructions do not depart from thespirit and scope of the invention in its broadest form.

[0025] Before undertaking the DETAILED DESCRIPTION OF THE INVENTIONbelow, it may be advantageous to set forth definitions of certain wordsand phrases used throughout this patent document: the terms “include”and “comprise,” as well as derivatives thereof, mean inclusion withoutlimitation; the term “or,” is inclusive, meaning and/or; the phrases“associated with” and “associated therewith,” as well as derivativesthereof, may mean to include, be included within, interconnect with,contain, be contained within, connect to or with, couple to or with, becommunicable with, cooperate with, interleave, juxtapose, be proximateto, be bound to or with, have, have a property of, or the like; and theterm “controller” means any device, system or part thereof that controlsat least one operation, such a device may be implemented in hardware,firmware or software, or some combination of at least two of the same.It should be noted that the functionality associated with any particularcontroller may be centralized or distributed, whether locally orremotely. Definitions for certain words and phrases are providedthroughout this patent document, those of ordinary skill in the artshould understand that in many, if not most instances, such definitionsapply to prior, as well as future uses of such defined words andphrases.

BRIEF DESCRIPTION OF THE DRAWINGS

[0026] For a more complete understanding of the present invention, andthe advantages thereof, reference is now made to the followingdescriptions taken in conjunction with the accompanying drawings,wherein like numbers designate like objects, and in which:

[0027]FIG. 1 illustrates an exemplary wireless network according to oneembodiment of the present invention;

[0028]FIG. 2 illustrates selected portions of an exemplary base stationaccording to one embodiment of the present invention;

[0029]FIG. 3 illustrates various exemplary transmit beams associatedwith different sectors of the exemplary base station according to oneembodiment of the present invention; and

[0030]FIG. 4 is a flow diagram illustrating the operation of theexemplary base station according to one embodiment of the presentinvention.

DETAILED DESCRIPTION OF THE INVENTION

[0031]FIGS. 1 through 4, discussed below, and the various embodimentsused to describe the principles of the present invention in this patentdocument are by way of illustration only and should not be construed inany way to limit the scope of the invention. Those skilled in the artwill understand that the principles of the present invention may beimplemented in any suitably arranged wireless network base station.

[0032]FIG. 1 illustrates exemplary wireless network 100 according to oneembodiment of the present invention. Wireless network 100 comprises aplurality of cell sites 121-123, each containing one of the basestations, BS 101, BS 102, or BS 103. Base stations 101-103 communicatewith a plurality of mobile stations (MS) 111-114 over, for example, codedivision multiple access (CDMA) channels. Mobile stations 111-114 may beany suitable wireless access terminals, including conventional cellularphones, PCS handset devices, personal digital assistants, portablecomputers, or metering devices. The present invention is not limited tomobile devices. Other types of wireless access terminals, includingfixed wireless terminals, may be used. However, for the sake ofsimplicity, only mobile stations are shown and discussed hereafter.

[0033] Dotted lines show the approximate boundaries of the cell sites121-123 in which base stations 101-103 are located. The cell sites areshown approximately circular for the purposes of illustration andexplanation only. It should be clearly understood that the cell sitesmay have other irregular shapes, depending on the cell configurationselected and natural and man-made obstructions.

[0034] As is well known in the art, cell sites 121-123 are comprised ofa plurality of sectors, each sector being illuminated by a directionalantenna coupled to the base station. Those acquainted with the art willrecognize that the coverage provided by the overhead signals (pilot,paging and synch channel) transmitted by each sector directional antennadetermines the sector geometry and coverage. Each sector of a basestation uses a unique short code (containing 2¹⁵ bits) as a modulationor spreading code to identify itself to access terminals. The embodimentof FIG. 1 illustrates the base station in the center of the cell.Alternate embodiments position the directional antennas in corners ofthe sectors. The system of the present invention is not limited to anyone cell site configuration.

[0035] In one embodiment of the present invention, BS 101, BS 102, andBS 103 comprise a base station controller (BSC) and one or more basetransceiver subsystem(s) (BTS). Base station controllers and basetransceiver subsystems are well known to those skilled in the art. Abase station controller is a device that manages wireless communicationsresources, including the base transceiver stations, for specified cellswithin a wireless communications network. A base transceiver subsystemcomprises the RF transceivers, antennas, and other electrical equipmentlocated in each cell site. This equipment may include air conditioningunits, heating units, electrical supplies, telephone line interfaces,and RF transmitters and RF receivers. For the purpose of simplicity andclarity in explaining the operation of the present invention, the basetransceiver subsystem in each of cells 121, 122, and 123 and the basestation controller associated with each base transceiver subsystem arecollectively represented by BS 101, BS 102 and BS 103, respectively.

[0036] BS 101, BS 102 and BS 103 transfer voice and data signals betweeneach other and the public switched telephone network (PSTN) (not shown)via communications line 131 and mobile switching center MSC 140. Line131 also provides the connection path to transfers control signalsbetween MSC 140 and BS 101, BS 102 and BS 103 used to establishconnections for voice and data circuits between MSC 140 and BS 101, BS102 and BS 103.

[0037] Communications line 131 may be any suitable connection means,including a T1 line, a T3 line, a fiber optic link, a network packetdata backbone connection, or any other type of data connection. Line 131links each vocoder in the BSC with switch elements in MSC 140. Thoseskilled in the art will recognize that the connections on line 131 mayprovide a transmission path for transmission of analog voice bandsignals, a digital path for transmission of voice signals in the pulsecode modulated (PCM) format, a digital path for transmission of voicesignals in an Internet Protocol (IP) format, a digital path fortransmission of voice signals in an asynchronous transfer mode (ATM)format, or other suitable connection transmission protocol. Thoseskilled in the art will recognize that the connections on line 131 may aprovide a transmission path for transmission of analog or digitalcontrol signals in a suitable signaling protocol.

[0038] MSC 140 is a switching device that provides services andcoordination between the subscribers in a wireless network and externalnetworks, such as the PSTN or Internet. MSC 140 is well known to thoseskilled in the art. In some embodiments of the present invention,communications line 131 may be several different data links where eachdata link couples one of BS 101, BS 102, or BS 103 to MSC 140.

[0039] In the exemplary wireless network 100, MS 111 is located in cellsite 121 and is in communication with BS 101. MS 113 is located in cellsite 122 and is in communication with BS 102. MS 114 is located in cellsite 123 and is in communication with BS 103. MS 112 is also locatedclose to the edge of cell site 123 and is moving in the direction ofcell site 123, as indicated by the direction arrow proximate MS 112. Atsome point, as MS 112 moves into cell site 123 and out of cell site 121,a hand-off will occur.

[0040] As is well known, the hand-off procedure transfers control of acall from a first cell site to a second cell site. As MS 112 moves fromcell 121 to cell 123, MS 112 detects the pilot signal from BS 103 andsends a Pilot Strength Measurement Message to BS 101. When the strengthof the pilot transmitted by BS 103 and received and reported by MS 112exceeds a threshold, BS 101 initiates a soft hand-off process bysignaling the target BS 103 that a handoff is required as described inTIA/EIA IS-95 or TIA/EIA IS-2000.

[0041] BS 103 and MS 112 proceed to negotiate establishment of acommunications link in the CDMA channel. Following establishment of thecommunications link between BS 103 and MS 112, MS 112 communicates withboth BS 101 and BS 103 in a soft handoff mode. Those acquainted with theart will recognize that soft hand-off improves the performance on bothforward (BS to MS) channel and reverse (MS to BS) channel links. Whenthe signal from BS 101 falls below a predetermined signal strengththreshold, MS 112 may then drop the link with BS 101 and only receivesignals from BS 103. The call is thereby seamlessly transferred from BS101 to BS 103.

[0042] The above-described soft hand-off assumes the mobile station isin a voice or data call. An idle hand-off is a hand-off of a mobilestation, between cells sites, that is communicating in the to control orpaging channel.

[0043]FIG. 2 illustrates selected portions of the base transceiversubsystem (BTS) of exemplary base station 101. According to anadvantageous embodiment of the present invention, base station 101 isdivided into three sectors, referred to arbitrarily as Sector A, SectorB, and Sector C. Each sector is covered by an adaptive antenna arraythat uses up to M antennas to form transmit beams that directionallytransmit voice and data from the base station to one or more mobilestations in the forward channel (i.e., downlink traffic). Base station101 comprises Sector A transceiver unit 210A, Sector B transceiver unit210B, and Sector C transceiver unit 210C, N channel element and CDMAunits 254, N adaptive antenna array (AAA) and beamforming (BF)controllers 252, resource management controller and database 260 andcall processing manager 270.

[0044] Sector A transceiver unit 210A, Sector B transceiver unit 210B,and Sector C transceiver unit 210C, N channel element and CDMA units254, N adaptive antenna array (AAA) and beamforming (BF) controllers 252operate like a conventional three sector, adaptive antenna array BTSwith respect to communicating with wireless access terminals (i.e.,mobile stations) in the forward channel using transmit beams. However,resource management controller and database 260 provides base station101 with unique and novel capabilities for using the same Walsh code (orother orthogonal code) to communicate simultaneously with two or morewireless access terminals within the same sector and in differentsectors of base station 101. Resource management controller and database260 comprises a processor and memory that execute an algorithm thatperforms resource management in the adaptive antenna array BTS of basestation 101. As will be explained below in greater detail, the algorithmis based on spatial isolation of mobile users which fall into differentdownlink beams in the same sector or in adjacent sectors of the sameBTS.

[0045] Since Sector B transceiver unit 210B and Sector C transceiverunit 210C are substantially similar to Sector A transceiver unit 210A,only Sector A transceiver unit 210A is illustrated and discussed indetail hereafter. Sector A transceiver unit 210A comprises Mtransceivers, including exemplary transceivers 215A, 215B, and 215C,which are arbitrarily labeled Transceiver M, Transceiver 2, andTransceiver 1, respectively. Since transceivers 215B and transceiver215C are substantially similar to transceiver 215A, only transceiver215A is illustrated and discussed in detail hereafter.

[0046] The transmit path of transceiver 215A comprises in-phase (I) andquadrature (Q) combiner block 222, Sector A I/Q modulator 224,up-converter and filter block 226, radio frequency (RF) amplifier 228,duplexer 230, and antenna 235. The receive path of transceiver 215Acomprises antenna 235, duplexer 230, low-noise amplifier (LNA) 240,down-converter and filter block 242, and Sector A demodulator 244.Compared to a prior art, non-adaptive BTS, the adaptive antenna array ofthe BTS of base station 101 employs multiple antennas 235 and multiple(up to M) transceiver units 210 and adaptive antenna array (AAA) andbeamforming (BF) controllers 252 to transmit directed beams in theforward channel (i.e., downlink).

[0047] In the reverse channel (uplink) from a mobile station (MS), thesignals received by the multiple antennas (antenna array) 235 areamplified by LNA 240, filtered and down-converted by down-converter andfilter block 242, and demodulated into digital in-phase (I) andquadrature (Q) streams by Sector A demodulator 244. Duplexer (DUP) 230provides isolation of transmitted and received signals. The digital Iand Q streams are fed to a CDMA modem for despreading and M-ary symboldetection. Beamforming controller 252 determines the beam formingcoefficients of the beamforming vector that describes the angle ofarrival and beam characteristics of the signal received from each mobileterminal.

[0048] During the uplink, adaptive antenna array and beamformingcontroller 252 estimates over several symbol periods the phase (i.e.,time offset) and signal strength of the received uplink signals at eachantenna element from each mobile station and determines uplink anddownlink beamforming (BF) weight vector coefficients for each mobilestation. Adaptive antenna array and beamforming controller 252 passesthe beamforming coefficient information to resource managementcontroller and database 260, which stores them in a database table.Reception of an access signal by the uplink on a specific sector andreceiver and detection circuit path is also identified to resourcemanagement controller and database 260. Resource management controllerand database 260 uses this information to assign the correspondingsector path for the downlink.

[0049] Resource management controller and database 260 communicates withcall processing manager 270 in order to assign a channel element, aWalsh code and a sector for each traffic channel established between theBTS and a mobile station. Resource management controller and database260 maintains a database in memory for the beamforming coefficients,idle/active state of each Walsh code, and the assignment of that Walshcode to an active channel. Each channel element and CDMA modem 254 iscapable of to supporting the signal processing for N users.

[0050] For the downlink to the wireless access terminal (i.e., mobilestation), the incoming I and Q data streams to the channel element arefirst processed in the CDMA modem, which selects the Walsh code (WC)according to the algorithm described in FIG. 4. The channel element andCDMA modem provides Walsh code modulation and PN code spreading on thedownlink. Next, the modem output is multiplied by a Mxl downlinkbeamforming weight vector for the mobile station in adaptive antennaarray and beamforming controller 252 and is distributed to M antenna 235for transmission in a given sector.

[0051] Adaptive antenna array and beamforming controller 252 performsamplitude weighting and phase shifting of the digital I and Q data foeach mobile station and conversion into Mxl vector form. I and Qcombiner 222 combines digital I and Q streams from N channel element andCDMA modem units 254. The combined I and Q signals from I and Q combiner222 are applied to Sector A I/Q modulator 224, which modulates a carriersignal. The modulated carrier signal is up-converted and filtered byup-converter and filter block 226, amplified by RF amplifier 228, andsent to each antenna element 235 via duplexer 230. Finally, the signalsat the antenna array are transmitted to the mobile station.

[0052]FIG. 3 illustrates various exemplary transmit beams transmitted byexemplary base station 101 into different sectors of cell site 121according to one embodiment of the present invention. Mobile stationsare represented by black dots in FIG. 3. Sector A contains threeexisting transmits beams, B1, B2, and B3. A new mobile station (NEW MS)that is accessing base station 101 is shown disposed within a new beam,B (New), to be formed by base station 101, as explained below in greaterdetail.

[0053]FIG. 4 depicts flow diagram 400, which illustrates the operationof exemplary base station 101 according to one embodiment of the presentinvention. Initially, resource management controller and database 260 isin an idle state, in which execution of the Walsh code (WC) allocationalgorithm is not required for resource assignment (process step 405). Atsome point, call processing manager 270 signals resource managementcontroller and database 260 to allocate resources for a traffic channel(process step 410). Next, resource management controller and database260 executes a hashing function or some other selection algorithm inorder to assign a physical channel element (CE) to the new mobilestation from the set of idle channel elements stored in resourcemanagement controller and database 260 (process 415).

[0054] Adaptive antenna array and beamforming controller 252 thenestimates the beamforming coefficients of the new mobile station fromthe reverse channel (i.e., uplink) signals for the new mobile station(process step 420). Resource management controller and database 260 thensearches the active Walsh codes and corresponding BF coefficients forthe sector and selects the Walsh code(s) whose BF weight vector(s) hasthe least correlation with the estimated BF weight vector of the newmobile station. Thus:

i=arg{min[|b*_(MS)b(i)|]}, for i=1, 2, 3, . . . Q;

WC_(MS)=WC(i );

[0055] where Q is the number of active users. If the search determinesthat a group of Walsh codes share the same BF coefficient, then resourcemanagement controller and database 260 select the Walsh code which isless assigned among currently active resources (process step 425).Resource management controller and database 260 then executes a hashingfunction or other selection algorithm to assign a Walsh code from theset of Walsh codes identified by resource management controller anddatabase 260 (process step 430).

[0056] Thereafter, base station 101 and resource management controllerand database 260 enter a Call Active state in which the channel element,the Walsh code, the BF weight vector, and the sector are all assigned(process step 435). A call softer handoff (i.e., a sector-to-sectorhandoff) causes resource management to controller and database 260 totest if the Walsh code is active in an adjacent sector of base station101 (process step 445). If the mobile station enters a softer handoffprocess, resource management controller and database 260 loads thedownlink BF weight vector of the mobile station in the handoff sensed byantenna array of the adjacent candidate sector (process step 450). Thealgorithm then loops back and executes the Walsh Code and BF weightsearch described for process step 425 using with the new BF weightvector.

[0057] Assuming no handoff occurs, base station 101 and the mobilestation continue communicating using the assigned Walsh code until acall release signal is received. If a call release signal is received,resource management controller and database 260 is notified to releaseand mark as idle the channel element (CE), the Walsh code (if not usedby another CE), and other sector resources for use by another call(process steps 455 and 460).

[0058] Returning now to FIG. 3, two different scenarios are considered.In the first scenario, the new (or candidate) mobile station (NEW MS) isnot in the softer handoff region and there are currently three (3)different beams (B1, B2, and B3) occupied by a number of active mobilestations. It is assumed that sector A of base station 101 is operatingwith all Walsh codes used to support traffic channels.

[0059] The new mobile station (NEW MS) requests service in Sector A. Thedownlink beamforming coefficients BNEW are estimated by adaptive antennaarray and beamforming controller 252 and algorithm described in FIG. 4is executed in base station 101. Resource management controller anddatabase 260 determines that B_(NEW) of NEW MS has the minimumcorrelation with the beamforming coefficients of beam B1. By way ofexample, assume that Walsh Codes (W20-W31, W33-W44) are used in beam B1.Starting from the first Walsh code in that group (i.e., WC20), resourcemanagement controller and database 260 searches for the Walsh code thatis least used and, when it finds a Walsh code that is used only once,that Walsh Code is assigned to NEW MS.

[0060] In the second scenario, NEW MS is located in the softer handoffregion between Sector A and Sector B. In this scenario, the newbeamforming weight vector of the candidate user (NEW MS) seen by SectorB is loaded and resource management controller and database 260 isnotified to execute a search algorithm within the new table for SectorB. In other words, for whichever sector to which the mobile station ishanded off, resource management controller and database 260 executes theWC allocation algorithm using the table for that sector.

[0061] The algorithm provided by the present invention relies on theminimum correlation criteria between downlink beams. This is becausesignal maximization is considered when constructing downlink beams.Therefore, multiple users may fall into the same beams. However, ifinterference nulling is considered instead of signal maximization, thealgorithm of the present invention needs modification such as usingcarrier-to-interference ratio (C/I) or some other measures as thecriteria when assigning Walsh Codes.

[0062] Although the present invention has been described in detail,those skilled in the art should understand that they can make variouschanges, substitutions and alterations herein without departing from thespirit and scope of the invention in its broadest form.

What is claimed is:
 1. For use in a base station of a code division multiple access (CDMA) wireless network, wherein said base station communicates with a plurality of wireless access terminals using transmit beams formed by an adaptive antenna array, an apparatus for allocating orthogonal codes used for downlink transmissions to said plurality of wireless access terminals comprising: a database capable of storing R active wireless terminal records, each of said R active wireless terminal records containing: 1) an active orthogonal code and 2) corresponding downlink beamforming coefficients used to communicate with one of said wireless access terminals; and a controller associated with said database capable of receiving a notification that a new wireless access terminal is accessing said base station and, in response to said notification, comparing said each of said R active wireless terminal records to new downlink beamforming coefficients suitable for forming a downlink transmit beam for transmitting to said new wireless access terminal and, in response to said comparison, determines at least one active wireless terminal record containing corresponding downlink beamforming coefficients that have the least correlation with said new downlink beamforming coefficients.
 2. The apparatus as set forth in claim 1 wherein said controller assigns an active orthogonal code in said at least one active wireless terminal record to be used in downlink transmissions to said new wireless access terminal.
 3. The apparatus as set forth in claim 2 wherein said base station uses up to K orthogonal codes for said downlink transmissions and said controller compares said each of said R active wireless terminal records to said new downlink beamforming coefficients in response to a determination that all of said K orthogonal codes are in use.
 4. The apparatus as set forth in claim 3 wherein said controller determines a first plurality of active wireless terminal records containing corresponding downlink beamforming coefficients that have the least correlation with said new downlink beamforming coefficients and further determines from said first plurality of active wireless terminal records a first active wireless terminal record containing an active orthogonal code used for downlink transmissions to a least number of said plurality of wireless access terminals.
 5. The apparatus as set forth in claim 4 wherein said controller assigns said active orthogonal code in said first active wireless terminal record to be used in downlink transmissions to said new wireless access terminal.
 6. The apparatus as set forth in claim 2 wherein said base station is operable to communicate in S sectors of a cell site associated with said base station and said base station uses up to K orthogonal codes in each of said S sectors for said downlink transmissions and wherein said controller compares said each of said R active wireless terminal records to said new downlink beamforming coefficients in response to a determination that all of said K orthogonal codes are in use in a first sector in which said new wireless access terminal is accessing said base station.
 7. The apparatus as set forth in claim 6 wherein said controller determines a first plurality of active wireless terminal records containing corresponding downlink beamforming coefficients that have the least correlation with said new downlink beamforming coefficients and further determines from said first plurality of active wireless terminal records a first active wireless terminal record containing an active orthogonal code used for downlink transmissions to a least number of said plurality of wireless access terminals.
 8. The apparatus as set forth in claim 7 wherein said controller assigns said active orthogonal code in said first active wireless terminal record to be used in downlink transmissions to said new wireless access terminal.
 9. The apparatus as set forth in claim 2 wherein said controller receives said new downlink beamforming coefficients from a beamforming controller that determines said new downlink beamforming coefficients from an uplink signal transmitted by said new wireless access terminal.
 10. The apparatus as set forth in claim 2 wherein said base station is operable to communicate in S sectors of a cell site associated with said base station and said new wireless access terminal is being handed off from a first sector of said cell site to a second sector of said cell site, wherein said each of said R active wireless terminal records are associated with said second sector and said controller receives said new downlink beamforming coefficients from active wireless terminal records associated with said first sector.
 11. A code division multiple access (CDMA) wireless network comprising a plurality of base stations, each of said base stations communicating with a plurality of wireless access terminals using transmit beams formed by an adaptive antenna array, wherein said each base station comprises: an apparatus for allocating orthogonal codes used for downlink transmissions to said plurality of wireless access terminals comprising: a database capable of storing R active wireless terminal records, each of said R active wireless terminal records containing: 1) an active orthogonal code and 2) corresponding downlink beamforming coefficients used to communicate with one of said wireless access terminals; and a controller associated with said database capable of receiving a notification that a new wireless access terminal is accessing said each base station and, in response to said notification, comparing said each of said R active wireless terminal records to new downlink beamforming coefficients suitable for forming a downlink transmit beam for transmitting to said new wireless access terminal and, in response to said comparison, determines at least one active wireless terminal record containing corresponding downlink beamforming coefficients that have the least correlation with said new downlink beamforming coefficients.
 12. The CDMA wireless network as set forth in claim 11 wherein said controller assigns an active orthogonal code in said at least one active wireless terminal record to be used in downlink transmissions to said new wireless access terminal.
 13. The CDMA wireless network as set forth in claim 12 wherein said base station uses up to K orthogonal codes for said downlink transmissions and said controller compares said each of said R active wireless terminal records to said new downlink beamforming coefficients in response to a determination that all of said K orthogonal codes are in use.
 14. The CDMA wireless network as set forth in claim 13 wherein said controller determines a first plurality of active wireless terminal records containing corresponding downlink beamforming coefficients that have the least correlation with said new downlink beamforming coefficients and further determines from said first plurality of active wireless terminal records a first active wireless terminal record containing an active orthogonal code used for downlink transmissions to a least number of said plurality of wireless access terminals.
 15. The CDMA wireless network as set forth in claim 14 wherein said controller assigns said active orthogonal code in said first active wireless terminal record to be used in downlink transmissions to said new wireless access terminal.
 16. The CDMA wireless network as set forth in claim 12 wherein said base station is operable to communicate in S sectors of a cell site associated with said base station and said base station uses up to K orthogonal codes in each of said S sectors for said downlink transmissions and wherein said controller compares said each of said R active wireless terminal records to said new downlink beamforming coefficients in response to a determination that all of said K orthogonal codes are in use in a first sector in which said new wireless access terminal is accessing said base station.
 17. The CDMA wireless network as set forth in claim 16 wherein said controller determines a first plurality of active wireless terminal records containing corresponding downlink beamforming coefficients that have the least correlation with said new downlink beamforming coefficients and further determines from said first plurality of active wireless terminal records a first active wireless terminal record containing an active orthogonal code used for downlink transmissions to a least number of said plurality of wireless access terminals.
 18. The CDMA wireless network as set forth in claim 17 wherein said controller assigns said active orthogonal code in said first active wireless terminal record to be used in downlink transmissions to said new wireless access terminal.
 19. The CDMA wireless network as set forth in claim 12 wherein said controller receives said new downlink beamforming coefficients from a beamforming controller that determines said new downlink beamforming coefficients from an uplink signal transmitted by said new wireless access terminal.
 20. The CDMA wireless network as set forth in claim 12 wherein said base station is operable to communicate in S sectors of a cell site associated with said base station and said new wireless access terminal is being handed off from a first sector of said cell site to a second sector of said cell site, wherein said each of said R active wireless terminal records are associated with said second sector and said controller receives said new downlink beamforming coefficients from active wireless terminal records associated with said first sector.
 21. For use in a base station of a code division multiple access (CDMA) wireless network, wherein the base station communicates with wireless access terminals using transmit beams formed by an adaptive antenna array, a method for allocating orthogonal codes used for downlink transmissions to wireless access terminals, the method comprising the steps of: storing R active wireless terminal records, each of the R active wireless terminal records containing: 1) an active orthogonal code; and 2) corresponding downlink beamforming coefficients used to communicate with one of the wireless access terminals; receiving a notification that a new wireless access terminal is accessing the base station; in response to the notification, comparing each of the R active wireless terminal records to new downlink beamforming coefficients suitable for forming a downlink transmit beam for transmitting to the new wireless access terminal; and in response to the comparison, determining at least one active wireless terminal record containing corresponding downlink beamforming coefficients that have the least correlation with the new downlink beamforming coefficients.
 22. The method as set forth in claim 21 further comprising the step of assigning an active orthogonal code in the at least one active wireless terminal record to be used in downlink transmissions to the new wireless access terminal.
 23. The method as set forth in claim 22 wherein the base station uses up to K orthogonal codes for the downlink transmissions and the step of comparing comprises the step of comparing each of the R active wireless terminal records to the new downlink beamforming coefficients in response to a determination that all of the K orthogonal codes are in use.
 24. The method as set forth in claim 23 further comprising the steps of: determining a first plurality of active wireless terminal records containing corresponding downlink beamforming coefficients that have the least correlation with the new downlink beamforming coefficients; and determining from the first plurality of active wireless terminal records a first active wireless terminal record containing an active orthogonal code used for downlink transmissions to a least number of the wireless access terminals. 